1. Field of the Invention
The present invention relates to an optical signal processing apparatus for processing signal light with multiple wavelengths.
2. Related Background Art
An optical signal processing apparatus separates multiplexed signal light with wavelengths into light components of the respective wavelengths and individually outputs the signal light components of the respective wavelengths, or multiplexes signal light components with different wavelengths, which are input for the respective wavelengths, and outputs signal light with multiple wavelengths. Alternatively, an optical signal processing apparatus separates signal light with multiple wavelengths into signal light components of the respective wavelengths, executes certain processing (e.g., loss impartation), then multiplexes the signal light components, and outputs signal light with multiple wavelengths. Such an optical signal processing apparatus is used in an optical communication system as an optical multiplexer, an optical demultiplexer, or a loss filter.
For example, an optical signal processing apparatus makes multiplexed signal light with wavelengths incident on a diffraction grating, separates the signal light into signal light components of the respective wavelengths using a fact that the optical diffraction angle of the diffraction grating changes depending on the wavelength, and outputs the separated signal light components of the respective wavelengths to spatially different optical paths. This optical signal processing apparatus imparts a predetermined loss to the spatially separated signal light components of the respective wavelengths, multiplexes the signal light components of the respective wavelengths, and outputs the multiplexed signal light with the wavelengths. That is, the optical signal processing apparatus operates as a loss filter. As an apparatus of this type, an optical signal processing apparatus disclosed in WO 01/04674A1 is known.
When the optical signal processing apparatus should perform processing for the signal light components of the respective wavelengths, the spatial interval between the optical paths of the signal light components of the respective wavelengths is preferably sufficiently large at the processing position. When the optical signal processing apparatus should multiplex or demultiplex wavelengths, the spatial interval between the optical paths of the signal light components of the respective wavelengths is preferably sufficiently large at the position where the signal light components are input or output for the respective wavelengths. In the latter case, if a fiber collimator is arranged at the position where the signal light components are input or output for the respective wavelengths, the spatial interval between the optical paths of the signal light components of the respective wavelengths must be larger than the outer diameter of the fiber collimator.
Let xcex9 be the grating period of a diffraction grating, xcex be the wavelength of light incident on the diffraction grating, xcex20 be the incident angle of light on the diffraction grating, and xcex21 be the diffraction angle. At this time, a relation given by
mxcex=xcex9(sin xcex20+sin xcex21)xe2x80x83xe2x80x83(1)
holds. In this case, m is the diffraction order. In a signal light wavelength band (1.55 xcexcm) used in optical communication, the diffraction angle difference with respect to the unit wavelength difference is as small as 0.1 deg/nm or less. Hence, when the interval between the optical paths of the signal light components of the respective wavelengths should be sufficiently large after wavelength separation by the diffraction grating, the optical path length from the diffraction grating increases. For this reason, a conventional optical signal processing apparatus is bulky.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention has been made to solve the above problem, and has as its object to provide an optical signal processing apparatus capable of size reduction.
An optical signal processing apparatus according to the present invention is characterized by comprising (1) wavelength separation means for receiving multiplexed signal light having multiple wavelengths xcex1 to xcexN (xcexnxe2x88x921 less than xcexn; N is an integer, 2xe2x89xa6N; n is an integer, 2xe2x89xa6nxe2x89xa6N), separating the multi-wavelength signal light into signal light components of respective wavelengths, and outputting the separated signal light components of the respective wavelengths to spatially different optical paths, and (2) optical path changing means for changing the optical paths of the signal light components of the respective wavelengths output from the wavelength separation means. The optical signal processing apparatus is also characterized in that for a first signal light component (wavelength xcexnxe2x88x921) and second signal light component (wavelength xcexn), which have adjacent wavelengths in the signal light components with the multiple wavelengths, letting G be a position at which the multi-wavelength signal light is wavelength-separated by the wavelength separation means, Pnxe2x88x921 be a position at which an optical path of the first signal light component is changed by the optical path changing means, Lnxe2x88x921 be a distance from the position G to the position Pnxe2x88x921 along the optical path of the first signal light component, and Pn be a position which is located on an optical path of the second signal light component from the position G to the optical path changing means or on an extended line of the optical path, and separated from the position G by the distance Lnxe2x88x921, a distance between the optical paths of the first and second signal light components, which are changed by the optical path changing means, is set to be larger than a distance between the position Pnxe2x88x921 and the position Pn.
In this optical signal processing apparatus, when the multiplexed signal light having multiple wavelengths xcex1 to xcexN is input to the wavelength separation means, the signal light of the multiple wavelengths is separated for the respective wavelengths and output to different optical paths. The optical paths of the signal light components of the respective wavelengths, which are wavelength-separated, are changed by the optical path changing means. With this optical path change, the optical path interval between the signal light components of the respective wavelengths is increased. The signal light components can also propagate in a reverse direction. Even when the degree of optical path separation for the signal light components of the respective wavelengths by the wavelength separation means is low, the optical path interval between the signal light components of the respective wavelengths is increased by the optical path changing means. Hence, the optical signal processing apparatus according to the present invention can be made compact because the optical path after optical path separation for the signal light components of the respective wavelengths by the wavelength separation means can be short.
The optical signal processing apparatus according to the present invention preferably includes a diffraction grating. In this case, letting xcex9 be a grating period of the diffraction grating, xcex8 be an angle made by a plane perpendicular to a grating direction of the diffraction grating and an incident direction of signal light on the diffraction grating, xcex2n be a diffraction angle when a traveling direction of the second signal light component, which is diffracted by the diffraction grating, is projected to the plane perpendicular to the grating direction, xcex2nxe2x88x921 be a diffraction angle when a traveling direction of the first signal light component, which is diffracted by said diffraction grating, is projected to the plane perpendicular to the grating direction and xcex94L be a distance from a diffraction position on the diffraction grating to a light-receiving position along the optical path of the second signal light component, a distance xcex94d between the light-receiving position of the second signal light component wavelength-separated by the diffraction grating and a light-receiving position of the first signal light component wavelength-separated by the diffraction grating satisfies
xcex94d greater than 2xcex94L cos xcex8 sin(|xcex2nxe2x88x92xcex2nxe2x88x921|/2).
In this case, the optical paths of the signal light components of the respective wavelengths are separated by the diffraction grating serving as the wavelength separation means. After that, the optical path interval between the signal light components of the respective wavelengths is increased such that the above inequality is satisfied. When the outer diameter of the fiber collimator used to input/output the signal light is taken into consideration, the distance xcex94d is preferably 1 mm or more. The distance xcex94d is more preferably 1.5 mm or more. Most preferably, the distance xcex94d is 3 mm or more.
In the optical signal processing apparatus according to the present invention, preferably, the wavelength separation means includes two diffraction gratings having identical structures, and the signal light is sequentially diffracted by the two diffraction gratings. In this case, the signal light with the multiple wavelengths is sequentially diffracted by the two diffraction gratings and wavelength-separated. Since the signal light components of the respective wavelengths travel in parallel to each other after the second diffraction, the optical signal processing apparatus can easily be assembled.
In the optical signal processing apparatus according to the present invention, the optical path changing means preferably includes a prism. In addition, the optical path changing means preferably includes an integrated member which changes the optical paths of the multi-wavelength signal light components. In this case, the optical paths of the wavelength-separated signal light components of the respective wavelengths are changed by the prism to increase the optical path interval.
In the optical signal processing apparatus according to the present invention, the optical path changing means preferably changes the optical paths of the signal light components of the respective wavelengths output from the wavelength separation means to a direction perpendicular to a predetermined plane including the optical paths of the signal light components of the respective wavelengths from the wavelength separation means to the optical path changing means. In this case, the optical elements that constitute the optical path changing means can easily be arranged, and the optical signal processing apparatus can easily be assembled.
In the optical signal processing apparatus according to the present invention, the optical path changing means preferably changes the optical paths so as to make an optical path of a signal light component having a wavelength xcex2p (p is an integer, 2xe2x89xa62pxe2x89xa6N) and that of a signal light component having a wavelength xcex2qxe2x88x921 (q is an integer, 1xe2x89xa62qxe2x88x921xe2x89xa6N) travel in different directions. At this time, the apparatus preferably further comprises first optical multiplex means for multiplexing the signal light components having the wavelengths xcex2p after the optical paths are changed by the optical path changing means and outputting the signal light and also further comprises second optical multiplex means for multiplexing the signal light components having the wavelengths xcex2qxe2x88x921 after the optical paths are changed by the optical path changing means and outputting the signal light. Alternatively, the optical path changing means preferably changes the optical path of the signal light component with the wavelength xcex2p to a direction perpendicular to the predetermined plane, then changes the optical path to a direction reverse to the direction of the optical path of the signal light component on the predetermined plane, and makes the signal light component incident on the wavelength separation means. In addition, the optical path changing means preferably changes the optical path of the signal light component with the wavelength xcex2qxe2x88x921 to a direction perpendicular to the predetermined plane, then changes the optical path to a direction reverse to the direction of the optical path of the signal light component on the predetermined plane, and makes the signal light component incident on the wavelength separation means. In this case, the optical signal processing apparatus can operate as an interleaver.